Taking advantage of the excellent weatherability due to chemical stability, the excellent chemical resistance and the excellent mechanical strength, polyolefin resins have been provided with desired physical properties and desired shapes in accordance with various molding processes such as the sheet molding, the film molding, the injection molding, the blow molding, the expansion molding, the vacuum molding and the rotation molding and have been widely used in various fields.
Recently, the decrease in the load on the environment is continuously required for every type of plastics due to the enhanced consciousness for protection of the environment. Since polyolefin-based resins have excellent properties for recycling, can be molded easily and do not emit harmful components during incineration, the polyolefin-based resins are attracting the attention as the most suitable material for decreasing the load on the environment. Therefore, it is estimated that the polyolefin-based resins are used more widely and required to have highly excellent properties.
To satisfy the above requirement, it is necessary that the workability in molding of the polyolefin resins is further improved and more excellent mechanical properties be exhibited. As a means to achieve these improvements, formation of composites has been attempted but is insufficient for improving the molding ability and exhibiting novel physical properties.
As the means for improving the molding ability, a process for producing a polyolefin-based resin using a polyene component is known. However, since the polyolefin-based resin is produced in accordance with a single stage polymerization process, the structure of the polymer is controlled essentially by adjusting the relative amounts of the monomer and the polyene used for the polymerization and there are no other effective means for the control. An increase in the amount of the polyene to improve the melting property is accompanied with frequent gelation and formation of insoluble products and, as the result; the improvement in the melting property is not achieved as desired. Moreover, a drawback arises in that, in accordance with the single stage polymerization process, the polyene cannot be copolymerized as desired in the effective range of the molecular weight distribution since the polyene is present in all ranges of the molecular weight distribution. For example, although it is necessary that the polyene be present in a small amount in the range of the high molecular weight to improve the melting property to a great degree, the amount of the polyene cannot be controlled in the desired manner. Therefore, the polyene must be used in an amount more than necessary to improve the melting property and frequent gelation and formation of insoluble products take place.
To overcome the above problem, multi-stage polymerization processes are proposed. In Japanese Patent Application Laid-Open Nos. Heisei 8(1996)-92337, Heisei 8(1996)-100036, Heisei 8(1996)-311136 and Heisei 9(1996)-235337, processes using polyenes for improving the balance between rigidity and impact strength of block copolymers containing propylene as the main component are disclosed. In these processes, propylene-based block copolymers are obtained by copolymerizing propylene and ethylene continuously after the homopolymerization of propylene. As the physical property of the resultant copolymer, the balance between rigidity and impact strength is improved. However, nothing is mentioned on the form of the reaction and the specific procedures of the process in the second stage that are important for the improvement in the property for melt molding. In the above processes, although the molecular weight of the block copolymer itself is mentioned, none of the control of the molecular weight, the selection of the catalyst in each stage and the type of the monomer in the second stage, which are the important factors for the improvement in the molding ability, are mentioned, either.
In WO 94/19382, a process for producing a propylene-based block copolymer using a diene is disclosed. As the characteristics of the process, an improvement in the balance between rigidity and impact resistance, an increase in the activation energy of melt flow, a decrease in the amount of gel components and the formation of an unsaturated group suitable for the macromolecular reaction are described.
However, the above process is limited to the production of a propylene-based block copolymer. Although the improvement in the melting property and the decrease in the amount of gel components are described, nothing is mentioned on the control of the multi-stage polymerization and, therefore, no remarkable improvement in the melting property can be expected. Nothing is mentioned on the uniform polymer composition in the above process.
Polypropylene has been widely used in various fields taking advantage of the excellent properties such as high rigidity, excellent heat resistance, lightweight, low price and excellent properties for the environment. However, it has been difficult due to the small tension in melted condition that applications are developed in the field of extrusion molding such as the sheet molding, the blow molding and the expansion molding. To improve the tension in melted condition, a process for obtaining a branched polypropylene by irradiation with electron beams is proposed (Japanese Examined Patent Publication No. 2655915). However, this process has drawbacks in that the facility is expensive and the production cost increases and that the resin is degraded (scission of molecular chains) in an extruder and the reuse of polypropylene becomes difficult.
As the process for producing polypropylene having a high melting elasticity, processes such as (i) a multi-stage polymerization (Japanese Patent Application Laid-Open No. Showa 58(1983)-219207), (ii) copolymerization with a polyene (Japanese Patent Application Laid-Open No. Heisei 8(1996)-92317), (iii) addition of an organic peroxide (Japanese Patent Application Laid-Open No. Heisei 7(1995)-138422), and (iv) irradiation with electron beams (Japanese Patent Application Laid-Open No. Showa 62(1987)-121704) are proposed. However, the process described in the foregoing item (i) has a drawback in that gel is formed due to tension in melted condition. The process described in the foregoing item (ii) has a drawback in that gel is formed and the secondary workability such as heat molding is adversely affected. The process described in the foregoing item (iii) has a drawback in that the handling property is poor and coloring and smelling take place. The process described in the foregoing item (iv) has a drawback in that the facility is expensive and the resin is degraded during recycling.